Chloroplast
A chloroplast is the green organelle in plant and algal cells where photosynthesis converts light energy, carbon dioxide, and water into sugar.
Definition
A chloroplast is a green part inside plant cells that captures sunlight and uses it to make food; it contains the green pigment chlorophyll, which is what makes leaves green. It is a membrane-bound organelle of plant and algal cells that carries out photosynthesis, with chlorophyll absorbing light energy, stacks of internal membranes called thylakoids where the light reactions occur, and a surrounding fluid, the stroma, where carbon dioxide is converted into glucose. Chloroplasts are plastids of cyanobacterial endosymbiotic origin, bounded by a double envelope, carrying their own circular DNA and ribosomes, with thylakoids stacked as grana. Photosystems II and I in the thylakoid membrane split water, build a proton gradient across the membrane, and reduce NADP$^+$ to NADPH while ATP synthase produces ATP. In the stroma the Calvin cycle uses that ATP and NADPH to fix $CO_2$ through the enzyme rubisco into three-carbon sugars.
Example
A leaf cell under a microscope shows many small green ovals, each a chloroplast making sugar whenever the sun shines. A single leaf mesophyll cell may hold about $30$ to $100$ chloroplasts, each around $5$ to $10\ \mu\text{m}$ long, and together the chloroplasts in one square meter of leaf can fix several grams of carbon dioxide on a sunny day. Rubisco is the most abundant protein on Earth but slow, fixing only about $3$ to $10$ $CO_2$ molecules per second per enzyme, so plants compensate by packing each chloroplast with vast quantities of it, up to half the soluble protein in a leaf.
Key Insight
Leaves look green because chlorophyll absorbs red and blue light and reflects green; it absorbs strongly near $430$ and $660\ \text{nm}$ and weakly around $550\ \text{nm}$, so the green you see is the light the plant did not use. Chloroplasts and mitochondria are partners, not opposites: chloroplasts store light energy in sugar, and mitochondria release that stored energy when the cell needs it. Nearly all the oxygen in the atmosphere was released by water-splitting in chloroplasts and their cyanobacterial ancestors.